This invention relates to the manufacture of gallium indium arsenide FET's, and in particular to the use of a structure suitable for self-aligned gate formation.
In the construction of FET's it is quite common to require the gate length to be as short as possible, and for some applications a gate length of less than one micron is desirable. In these circumstances mask registration becomes very severe if recourse is not to be had to some form of self-aligning technique. In the case of gallium arsenide FET's such a technique has been described by S. Umebachi et al in a letter entitled "A New Heterojunction Gate GaAs FET" appearing in `IEEE Transactions on Electron Devices, Vol. ED-22, pp. 613-614 (August, 1974)`.
In a gallium arsenide FET a self-aligned gate structure is provided by making the gate from twin layers, with a lower layer of p-type gallium aluminum arsenide covered by an upper layer of p-type gallium arsenide. Use is then made of an etch which will attack the gallium aluminum arsenide but not the gallium arsenide in order to undercut the upper layer and produce a `mushroom` whose overhang serves to separate a subsequent metallization into electrically isolated source, gate, and drain regions. However, no direct analogous structure could be found for a gallium indium arsenide FET having a composition providing a lattice constant substantially matched with that of indium phosphide. The provision of the gallium aluminum arsenide layer of the prior art FET relies upon the existence of selective etches that will produce undercutting by virtue of etching gallium aluminum arsenide while leaving gallium arsenide essentially free from attack. It is also advantageous that the gallium aluminum arsenide has a higher band gap than that of the underlying material of the channel of the FET. For a gallium indium arsenide FET we have failed to find a material that we can selectively etch in this undercutting way to leave the underlying gallium indium arsenide layer of the opposite conductivity type substantially unaffected.